Substantial interest exists in the manufacturing of so-called single-electron transistors which, among other things, have great potential for use in integrated circuits. A precondition for the realization of a so-called single electron transistor is the provision of a quantum dot, i.e. of a region in which the charge carriers are surrounded in all directions by potential barriers and which have quantized energy levels.
Through the use of single electron transistors it is possible to realize novel logic and memory circuits which make it possible to carry out complicated combinations of data which are not possible in standard logic circuits. Furthermore, the possibility exists of storing and processing individual bits in the form of single electrons. In addition, applications in electrometers and detectors are possible. The spectrum of use thus extends from large computers to the mobile telephone. The market potential of individual electron transistors can be seen from the fact that all major companies in the semiconductor industries worldwide are active in this field and that practically all kinds of integrated circuits (ICs) can be considered as an area of use.
For the industrial application, the active quantum dots in individual electron transistors must satisfy the following definitive requirements:
A. They should be manufacturable by a process which is as simple as possible and which is built up essentially of standard process steps of component manufacture and they should be combinable with the customary semiconductor technology, i.e. integratable into complex circuits. PA1 B. The active quantum dots should be capable of being controlled by only one gate because in this way a maximum packing density can be achieved on the chip. PA1 C. The contacting of the feedlines to the quantum dots should be capable of being realized simply. PA1 D. The structure size of the quantum dots should lie in the nanometer range, with the manufacture being reproducible. The boundary surfaces of the active quantum dots should be atomically smooth so that the individual electrons and transistors can also be operated at higher temperatures, i.e. at temperatures above 77.degree. K., and preferably at ambient temperature. PA1 (1) Y. Nagamune et al. PA1 (2) R. P. Taylor et al. PA1 (3) T. Fujisawa et al. PA1 (4) Y. Takahashi et al.
Having regard to the prior art, reference is made to the following publications:
Single electron transport and current quantization in a novel quantum dot structure PA2 Fabrication of nanostructures with multilevel architecture PA2 AlGaAs/InGaAs/GaAs single electron transistors fabricated by focused ion beam implantation PA2 Conductance Oscillations of a Si Single Electron Transistors at Room Temperature.
Summarizing, the following can be said with regard to the points A) to D):
With respect to A): No method has hitherto been known which permits the defined manufacture of quantum dots without the use of electron beam lithography or FIB plants (FIB=Focused Ion Beam). These methods are, on the one hand, not standard methods for the manufacture of components and, on the other hand, the use of this method is always associated with damage to the material as is evident from the documents (1), (2) and (3).
With regard to B): As a rule, the active quantum dots of individual electrons and transistors are defined by the application of voltage to several gates, whereby the space requirement and the susceptibility of the structures to breakdown increases greatly. Furthermore, it is technologically very difficult to stabilize the numerous gate voltages. In this connection, reference is again made to the documents (1), (2) and (3).
With regard to C): The contacting of the active quantum dots has not been satisfactorily solved for other non-semiconducting material systems.
With regard to D): The previously known methods achieve at a maximum a smoothness of the structures, such as can be achieved with the manufacturing methods that are used, and, in this case, reference is also made here to the documents (1), (2) and (3).